Process for recovering benzene and fuel gas in an aromatics complex

ABSTRACT

A process for separating xylenes from a feedstock in which the feedstock is separated into a xylene stream, a benzene rich stream and a light ends stream. Two separation zones may be utilized in which liquid from the first zone sent to a toluene recovery zone and vapor from the first zone is sent to a compression zone. The compressed vapor from the compression zone is sent to the second separation zone.

FIELD

This invention relates to an improved process for energy savings in thedistillation of hydrocarbons. More specifically, the present inventionconcerns energy conservation within an aromatics-processing complexproducing xylene isomers, benzene and fuel gas.

BACKGROUND

The xylene isomers are produced in large volumes from petroleum asfeedstocks for a variety of important industrial chemicals. The mostimportant of the xylene isomers is para-xylene, the principal feedstockfor polyester, which continues to enjoy a high growth rate from largebase demand. Ortho-xylene is used to produce phthalic anhydride, whichsupplies high-volume but relatively mature markets. Meta-xylene is usedin lesser, but growing, volumes for products such as plasticizers, azodyes and wood preservers. Ethylbenzene generally is present in xylenemixtures and is occasionally recovered for styrene production, but isusually considered a less-desirable component of C8 aromatics.

Among the aromatic hydrocarbons, the overall importance of xylenesrivals that of benzene as a feedstock for industrial chemicals. Xylenesand benzene are produced from petroleum by reforming naphtha, but not insufficient volume to meet demand; thus, conversion of other hydrocarbonsis necessary to increase the yield of xylenes and benzene. Often tolueneis de-alkylated to produce benzene or selectively disproportionated toyield benzene and C8 aromatics from which the individual xylene isomersare recovered.

Aromatics complexes producing xylenes are substantial consumers ofenergy, notably in distillation operations to prepare feedstocks andseparate products from conversion processes. The separation of xylenesfrom a feedstock in particular offers substantial potential for energysavings. Energy conservation in such processes would not only reduceprocessing costs, but also address current concerns about carbonemissions.

In addition to producing xylenes, valuable fuel gas is generated duringthe catalytic conversion of xylenes in an aromatics complex. A portionof this fuel gas is recoverable in a xylene isomerization unit.

The xylene isomerization units typically include a deheptanizer and astabilizer. The current designs for xylene isomerization units utilizeat least two recycle loops between the deheptanizer and the stabilizer.

In the first recycle loop, at least a portion of the overhead vapor fromthe stabilizer is recycled back to the deheptanizer. This will result inthis portion of the vapor being re-condensed, re-flashed, and ultimatelyre-compressed in the same separation process since it is passed back tothe deheptanizer and will pass through the same separation equipment.

The second recycle loop is formed between a receiver and a vent drumwhere the chilled liquid from the vent drum enters the hotter, lowpressure receiver and re-flashes into vapor to the compressor. Thisrequires the same compounds to be re-compressed and cooled once again.

It is believed that the current design is inefficient at least becauseboth of the recycle loops lead to undesirable re-processing of the samematerial at the expense of equipment capacity and utility cost. Energyconservation in such processes would not only reduce processing costsbut also would address current concerns about carbon emissions.

Therefore, there is a need to provide a xylene isomerization processwhich may be carried out more efficiently.

SUMMARY

The present disclosure describes a process related to an improvedprocess for energy savings in the distillation of hydrocarbons. Thisdisclosure eliminates the need for a stabilization zone and a secondseparation zone, allowing the first liquid stream, which is mostlytoluene, can be sent to toluene recovery. This eliminates the need forcostly extraction.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.Other objects, advantages and applications of the present invention willbecome apparent to those skilled in the art from the following detaileddescription and drawing. Additional objects, advantages and novelfeatures of the examples will be set forth in part in the descriptionwhich follows, and in part will become apparent to those skilled in theart upon examination of the following description and the accompanyingdrawing or may be learned by production or operation of the examples.The objects and advantages of the concepts may be realized and attainedby means of the methodologies, instrumentalities and combinationsparticularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

The FIG. 1 is simplified process flow diagram in which the FIGURE showsa xylene isomerate recovery process according to one or more embodimentsof the present invention.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the application and uses of the embodimentdescribed. Furthermore, there is no intention to be bound by any theorypresented in the preceding background or the following detaileddescription.

The description of the apparatus of this invention is presented withreference to the attached FIGURE. The FIG. 1 is a simplified diagram ofthe preferred embodiment of this invention and is not intended as anundue limitation on the generally broad scope of the descriptionprovided herein and the appended claims. Certain hardware such asvalves, pumps, compressors, heat exchangers, instrumentation andcontrols, have been omitted as not essential to a clear understanding ofthe invention. The use and application of this hardware is well withinthe skill of the art.

A process has been developed for separating xylene from a feedstock. Thefeedstock to the present process generally comprises alkylaromatichydrocarbons of the general formula C6H(6-n)Rn, where n is an integerfrom 0 to 5 and each R may be CH3, C2H5, C3H7, or C4H9, in anycombination. The aromatics-rich feedstock to the process of theinvention may be derived from a variety of sources, including withoutlimitation catalytic reforming, steam pyrolysis of naphtha, distillatesor other hydrocarbons to yield light olefins and heavier aromatics-richbyproducts (including gasoline-range material often referred to as“pygas”), and catalytic or thermal cracking of distillates and heavyoils to yield products in the gasoline range. Products from pyrolysis orother cracking operations generally will be hydrotreated according toprocesses well known in the industry before being charged to the complexin order to remove sulfur, olefins and other compounds which wouldaffect product quality and/or damage catalysts or adsorbents employedtherein. Light cycle oil from catalytic cracking also may bebeneficially hydrotreated and/or hydrocracked according to knowntechnology to yield products in the gasoline range; the hydrotreatingpreferably also includes catalytic reforming to yield the aromatics-richfeed stream. If the feedstock is catalytic reformate, the reformerpreferably is operated at high severity to achieve high aromatics yieldwith a low concentration of nonaromatics in the product.

The FIGURE shows a simplified flow diagram of a xylene isomeraterecovery portion of an aromatics-processing complex for a feedstockwhich typically contains olefinic compounds and light ends, e.g.,butanes and lighter hydrocarbons and such as pentanes, as well asbenzene, toluene and C8 aromatics and higher aromatics and aliphatichydrocarbons including naphthenes.

The feedstock is introduced via a line 10 to a xylene separation zone12. The xylene separation zone 12 includes a column 14. Preferably thecolumn 14 is a distillation column, and most preferably a deheptanizer.As will be appreciated by one of ordinary skill in the art, such acolumn 14 may contain trays or mechanical packing.

In the xylene separation zone 12, the feedstock is separated into avapor phase and a liquid phase. The liquid phase contains most of thexylene as well as other heavier hydrocarbons. The liquid phase may berecovered from the xylene separation zone 12 via a line 16 and passed tofurther processing units to recover the desired xylene isomer and othervaluable byproducts. The further processing units are known to those ofordinary skill in the art and are not necessary for a full understandingof the present invention.

The vapor phase in the xylene separation zone 12 contains hydrocarbonswith seven carbon atoms or less, benzene, and toluene. As will beappreciated by those of ordinary skill in the art, when separatinghydrocarbons, there are equilibrium distributions of components in vaporand liquid streams in close contact during the separation processes andthus, the present invention is intended to accommodate a range ofaromatic and non-aromatic component purities.

The vapor phase is recovered from the xylene separation zone 12 andpassed via a line 18 to a first separation zone 20. Within the firstseparation zone 20, the vapor phase is partially condensed. Accordingly,a first condenser 24 is provided in the line 18 and is used to pass thevapor phase from the xylene separation zone 12 to the first separationzone 20.

The first separation zone 20 typically includes a first vessel 22.Preferably the first vessel 22 of the first separation zone 20 has atemperature between approximately 32° C. to 149° C. In the first vessel22, the mixed vapor and liquid phases from first condenser 24 areseparated into a first liquid phase 28 and a first vapor phase 33. Thefirst liquid phase 28 comprises benzene and toluene as well as solublelevels of hydrocarbons with five carbons or less dissolved into theliquid phase. The first vapor phase 33 comprises mostly hydrogen, andhydrocarbons with four carbons or less. Again, there will be somecrossover amounts of compounds. A portion of the first liquid phase 28from the first separation zone 20 may be recycled to the xyleneseparation zone 12 via a line 30. Another portion of the first liquidphase 28 from the first separation zone 20 may be passed a toluenerecovery zone 40 via line 28.

The first vapor phase from the first separation zone 20 is passed via aline 33 to a compression zone 34 in which the first vapor phase iscompressed into a compressed vapor phase. The compression zone 34includes at least a compressor 36. From the compression zone 34, thecompressed and partially condensed first vapor phase is passed via aline 38 to the toluene recovery zone 40 which typically includes atoluene recovery unit 26. The toluene recovery unit 26 inputs includethe first liquid phase 28 and a benzene rich stream 28. The toluenerecovery unit 26 outputs include a benzene rich overhead stream 42 and atoluene rich bottoms stream 50.

In the first separation zone 20, the first liquid phase contains mostlytoluene. To avoid an energy penalty associated with re-extraction oftoluene, the first liquid phase is directed to toluene recovery insteadof a stabilization zone and a second separation zone before the toluenerecovery zone. The toluene recovery may be realized in a stripper columncommonly found a transalkylation unit thereby effecting a degree ofstabilization of the first liquid phase as well. As discussed above,vapor from the first separation zone 20 is sent to the compression zone34. The compressed and partially condensed vapor phase is directed tothe toluene recovery zone 40 via a line 38. A benzene-rich first liquidphase 28 may also be combined with another benzene rich stream from, forexample, an aromatic transalkylation unit via a line 48.

In the present invention, a second separation zone 60 is located afterthe toluene recovery zone 40. The second separation zone 60 includes acondenser 44 and a vessel 46 to separate the compressed and partiallycondensed vapor phase into a second vapor phase product removed via line52 and a benzene rich second liquid phase removed via lines 54, 56.

In the prior art, the combined liquid is sent to a stabilization zone ora second separation zone before being sent to a toluene recovery zone.However, in the present invention, there is no stabilization zone orsecond separation zone before the toluene recovery zone. In the presentinvention, the combined liquid stream is sent to toluene recoverydirectly. Unlike the prior art, the first liquid stream, which is mostlytoluene, doesn't have to be sent to a stabilization zone, where theliquid goes on to costly extraction.

The second vapor phase comprises a light ends vapor stream whichincludes hydrocarbons having four carbon atoms or less and hydrogen. Thelight ends vapor stream may be recovered from the second separation zone60 via a line 52, and, for example, may be passed back to a reactionzone (not shown), may be used as fuel, or be used for other processes.In the toluene recovery zone 40, the compressed and partially condensedfirst vapor phase is separated via vessel 46 into a second liquid phase54, 56 and a second vapor phase 52. The second liquid phase extends fromthe vessel 46. The temperature of the second separating zone 60 istypically between approximately 10° C. to 149° C.

The present invention avoids wasted energy caused by unnecessaryre-extraction of toluene. The benzene in the first vapor phase 33 fromthe first separation zone 20 and the benzene co-boiling species areconcentrated and sent directly to extraction from the compression zone34 while the toluene-rich first liquid phase 28 is sent directly totoluene recovery.

As will be appreciated, a process according to one or more of theseembodiments provides an effective and efficient process to separatexylene from a reaction effluent. More specifically, in one or moreembodiments of the present invention, stabilization zone is eliminatedsuch that the second liquid phase is sent directly to extraction.Unnecessary re-compression of overhead vapor from the stabilization zoneis eliminated.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not inteanded to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

While the invention has been described with what are presentlyconsidered the preferred embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments, but it isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

Specific Embodiments

While the following is described in conjunction with specificembodiments, it will be understood that this description is intended toillustrate and not limit the scope of the preceding description and theappended claims.

A first embodiment of the invention is a process for the recovery of abenzene rich liquid stream and a light ends vapor stream in a xyleneisomerization process from a feedstock, the process comprising passing afeedstock into a deheptanizer in which the feedstock is separated into adeheptanizer vapor phase and a deheptanizer liquid phase, thedeheptanizer vapor phase containing hydrocarbons with seven carbon atomsor less, and the deheptanizer liquid phase containing hydrocarbons witheight carbon atoms or more; passing the deheptanizer vapor phase fromthe deheptanizer to a first separation zone; separating the deheptanizervapor phase in the first separation zone into a first liquid phase and afirst vapor phase; passing the first liquid phase from the firstseparation zone to a toluene recovery comprising an overhead stream anda bottoms stream; passing the first vapor phase from the firstseparation zone to a compression zone in which the first vapor phase iscompressed and partially condensed to provide a compressed and partiallycondensed vapor phase; passing the compressed and partially condensedvapor phase from the compression zone to the toluene recovery zone to becombined with the toluene recovery zone overhead stream, which is passedto a second separation zone; separating the compressed and partiallycondensed vapor phase into a second liquid phase and a second vaporphase in the second separation zone, the second vapor phase being alight ends vapor stream; recovering the light ends vapor stream; passingthe second liquid phase back to the toluene recovery zone; andrecovering the benzene rich liquid stream. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the first embodiment in this paragraph further comprisingpassing a portion of the first liquid phase to the deheptanizer. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph furthercomprising passing the benzene rich liquid stream to an extraction zoneto recover a benzene stream. An embodiment of the invention is one, anyor all of prior embodiments in this paragraph up through the firstembodiment in this paragraph further comprising passing the benzene richliquid stream to storage. An embodiment of the invention is one, any orall of prior embodiments in this paragraph up through the firstembodiment in this paragraph further comprising compressing thedeheptanizer vapor stream before the deheptanizer vapor stream entersthe first separation zone. An embodiment of the invention is one, any orall of prior embodiments in this paragraph up through the firstembodiment in this paragraph wherein a temperature of the firstseparation zone is between approximately 32° C. to 149° C. An embodimentof the invention is one, any or all of prior embodiments in thisparagraph up through the first embodiment in this paragraph wherein atemperature of the second separation zone is between approximately 10°C. to 149° C. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph wherein a temperature of the first separation zone is betweenapproximately 10° C. to 149° C. and a temperature of the secondseparation zone is between approximately 10° C. to 149° C.

A second embodiment of the invention is a process for the recovery of abenzene rich liquid stream and a light ends vapor stream in a xyleneisomerization process from a feedstock, the process comprising passing afeedstock into a distillation column in which the feedstock is separatedinto a distillation vapor and a distillation liquid, the distillationvapor containing mostly hydrocarbons with seven carbon atoms or less;condensing the distillation vapor; separating the distillation vaporinto a first liquid phase and a first vapor phase; passing the firstliquid phase to a toluene recovery zone; compressing the first vaporphase into a compressed and partially condensed vapor phase; separatingthe compressed vapor phase into a second liquid phase and a second vaporphase, the second vapor phase being a light ends vapor stream;recovering the light ends vapor stream; passing the second liquid phaseto the toluene recovery zone; and recovering the benzene rich liquidstream. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the second embodiment in thisparagraph wherein a temperature of the first liquid phase is betweenapproximately 32° C. to 149° C. An embodiment of the invention is one,any or all of prior embodiments in this paragraph up through the secondembodiment in this paragraph wherein a temperature of the compressedvapor phase is between approximately 10° C. to 149° C. An embodiment ofthe invention is one, any or all of prior embodiments in this paragraphup through the second embodiment in this paragraph further comprisingpassing a portion of the first liquid phase to the distillation column.An embodiment of the invention is one, any or all of prior embodimentsin this paragraph up through the second embodiment in this paragraphfurther comprising passing the benzene rich liquid stream to anextraction zone to recover a benzene stream. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the second embodiment in this paragraph further comprisingpassing the benzene rich liquid stream to storage.

A third embodiment of the invention is a process for the recovery of abenzene rich liquid stream and a light ends vapor stream in a xyleneisomerization process from a feedstock, the process comprising passing afeedstock into a separation zone in which the feedstock is separatedinto a vapor stream and a liquid stream, the vapor stream containingmostly hydrocarbons with seven carbon atoms or less, the liquid streamcomprising xylene; condensing the vapor stream; separating the vaporstream into a first liquid phase and a first vapor phase; passing thefirst vapor phase to a compression zone; compressing the first vaporphase into a compressed and partially condensed vapor phase; separatingthe compressed vapor phase into a second liquid phase and a second vaporphase, the second vapor phase being a light ends vapor stream;recovering the light ends vapor stream; and recovering the benzene richliquid stream. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the third embodiment inthis paragraph wherein a temperature of the first liquid phase isbetween approximately 32° C. to 149° C. An embodiment of the inventionis one, any or all of prior embodiments in this paragraph up through thethird embodiment in this paragraph wherein a temperature of thecompressed vapor phase is between approximately 10° C. to 149° C.

Without further elaboration, it is believed that using the precedingdescription that one skilled in the art can utilize the presentinvention to its fullest extent and easily ascertain the essentialcharacteristics of this invention, without departing from the spirit andscope thereof, to make various changes and modifications of theinvention and to adapt it to various usages and conditions. Thepreceding preferred specific embodiments are, therefore, to be construedas merely illustrative, and not limiting the remainder of the disclosurein any way whatsoever, and that it is intended to cover variousmodifications and equivalent arrangements included within the scope ofthe appended claims.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.

The invention claimed is:
 1. A process for the recovery of a benzenerich liquid stream and a light ends vapor stream in a xyleneisomerization process from a feedstock, the process comprising: passinga feedstock into a deheptanizer in which the feedstock is separated intoa deheptanizer vapor phase and a deheptanizer liquid phase, thedeheptanizer vapor phase containing hydrocarbons with seven carbon atomsor less, and the deheptanizer liquid phase containing hydrocarbons witheight carbon atoms or more; passing the deheptanizer vapor phase fromthe deheptanizer to a first separation zone; separating the deheptanizervapor phase in the first separation zone into a first liquid phase and afirst vapor phase; passing the first liquid phase from the firstseparation zone to a toluene recovery zone, the toluene recovery zonecomprising a vessel configured to provide an overhead stream and abottoms stream; passing the first vapor phase from the first separationzone to a compression zone in which the first vapor phase is compressedand partially condensed to provide a compressed and partially condensedvapor phase; combining the compressed and partially condensed vaporphase from the compression zone with the toluene recovery zone overheadstream; separating the compressed and partially condensed vapor phaseand the toluene recovery zone overhead stream into a second liquid phaseand a second vapor phase in a second separation zone, the second vaporphase being a light ends vapor stream; recovering the light ends vaporstream; passing a portion of the second liquid phase back to the toluenerecovery zone; and recovering a toluene rich liquid stream from thetoluene recovery zone, wherein the toluene recovery zone comprises astripper column; and recovering the benzene rich liquid stream from thesecond liquid phase.
 2. The process of claim 1 further comprisingpassing a portion of the first liquid phase to the deheptanizer.
 3. Theprocess of claim 1 further comprising passing the benzene rich liquidstream to an extraction zone to recover a benzene stream.
 4. The processof claim 1 further comprising passing the benzene rich liquid stream tostorage.
 5. The process of claim 1 further comprising compressing thedeheptanizer vapor stream before the deheptanizer vapor stream entersthe first separation zone.
 6. The process of claim 5 wherein atemperature of the first separation zone is between approximately 32° C.to 149° C.
 7. The process of claim 6 wherein a temperature of the secondseparation zone is between approximately 10° C. to 149° C.
 8. Theprocess of claim 1 wherein a temperature of the first separation zone isbetween approximately 10° C. to 149° C. and a temperature of the secondseparation zone is between approximately 10° C. to 149° C.
 9. A processfor the recovery of a benzene rich liquid stream and a light ends vaporstream in a xylene isomerization process from a feedstock, the processcomprising: passing a feedstock into a distillation column in which thefeedstock is separated into a distillation vapor and a distillationliquid, the distillation vapor containing mostly hydrocarbons with sevencarbon atoms or less; condensing the distillation vapor; separating thedistillation vapor into a first liquid phase and a first vapor phase;passing the first liquid phase to a toluene recovery zone, wherein thetoluene recovery zone comprises a stripper column; compressing the firstvapor phase into a compressed and partially condensed vapor phase;separating the compressed vapor phase into a second liquid phase and asecond vapor phase, the second vapor phase being a light ends vaporstream; recovering the light ends vapor stream; passing the secondliquid phase to the toluene recovery zone; and recovering a toluene richliquid stream from the toluene recovery zone; and recovering the benzenerich liquid stream from the second liquid phase.
 10. The process ofclaim 9 wherein a temperature of the first liquid phase is betweenapproximately 32° C. to 149° C.
 11. The process of claim 10 wherein atemperature of the compressed vapor phase is between approximately 10°C. to 149° C.
 12. The process of claim 9 further comprising passing aportion of the first liquid phase to the distillation column.
 13. Theprocess of claim 9 further comprising passing the benzene rich liquidstream to an extraction zone to recover a benzene stream.
 14. Theprocess of claim 9 further comprising passing the benzene rich liquidstream to storage.
 15. A process for the recovery of a benzene richliquid stream and a light ends vapor stream in a xylene isomerizationprocess from a feedstock, the process comprising: passing a feedstockinto a separation zone in which the feedstock is separated into a vaporstream and a liquid stream, the vapor stream containing mostlyhydrocarbons with seven carbon atoms or less, the liquid streamcomprising xylene; condensing the vapor stream; separating the vaporstream into a first liquid phase and a first vapor phase; passing thefirst vapor phase to a compression zone; compressing the first vaporphase into a compressed and partially condensed vapor phase; separatingthe compressed vapor phase into a second liquid phase and a second vaporphase, the second vapor phase being a light ends vapor stream;recovering the light ends vapor stream; and recovering a toluene richliquid stream from a toluene recovery zone, wherein the toluene recoveryzone comprises a stripper column; and recovering the benzene rich liquidstream from the second liquid phase.
 16. The process of claim 15 whereina temperature of the first liquid phase is between approximately 32° C.to 149° C.
 17. The process of claim 15 wherein a temperature of thecompressed vapor phase is between approximately 10° C. to 149° C. 18.The process of claim 1, further comprising: recovering a first portionof the second liquid phase from the second separation zone; and, passinga second portion of the second liquid phase from the second separationzone to the toluene recovery zone.